Cellulose and hemicellulose are the most abundant plant materials produced by photosynthesis. They can be degraded and used as an energy source by numerous microorganisms, including bacteria, yeast and fungi, that produce extracellular enzymes capable of hydrolysis of the polymeric substrates to monomeric sugars (Aro et al., J. Biol. Chem., 10.1074/M003624200, Apr. 13, 2001). As the limits of non-renewable resources approach, the potential of cellulose to become a major renewable energy resource is enormous (Krishna et al., Bioresource Tech. 77:193-196, 2001). The effective utilization of cellulose through biological processes is one approach to overcoming the shortage of foods, feeds, and fuels (Ohmiya et al., Biotechnol. Gen. Engineer. Rev. 14:365-414, 1997).
Cellulose is a linear polysaccharide of glucose residues connected by β-1,4 linkages. In nature, cellulose is usually associated with lignin together with hemicelluloses such as xylans and glucomannans. The practical use of cellulases has been hampered by the nature of the known cellulases, which are often mixtures of cellulases having a variety of activities and substrate specificities. For that reason, it is desirable to identify cellulases having only the desired activities or proteins that may facilitate cellulase action.
Hemicellulose is one of any of several heteropolymers (matrix polysaccharides) present in almost all cell walls along with cellulose. Their molecular weights are usually lower than that of cellulose and they have a weak undifferentiated structure compared to crystalline cellulose. But the chains form a ‘ground’—they bind with pectin to cellulose to form a network of cross-linked fibers. Thus, it would be beneficial to enhance hemicellulose degradation.
O-Glycosyl hydrolases (EC 3.2.1.-) are a widespread group of enzymes that hydrolyse the glycosidic bond between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of up to 60 different families [HENRISSAT, B. AND BAIROCH, A. New families in the classification of glycosyl hydrolases based on amino acid sequence similarities. BIOCHEM. J. 293 781-788 (1993); HENRISSAT, B. A classification of glycosyl hydrolases based on amino acid sequence similarities. BIOCHEM. J. 280 309-316 (1991); DAVIES, G. AND HENRISSAT, B. Structures and mechanisms of glycosyl hydrolases. STRUCTURE 3 853-859 (1995); and HENRISSAT, B. AND BAIROCH, A. Updating the sequence-based classification of glycosyl hydrolases. BIOCHEM. J. 316 695-696 (1996)]. Acetyl xylan esterases (EC 3.1.1.72) are a group of enzymes that remove acetyl side groups from xylan. A classification system for carbohydrate esterases, based on sequence similarity, has led to the definition of 13 families, seven of which contain acetyl xylan esterases (COUTINHO, P. M. AND HENRISSAT, B., 1999 Carbohydrate-active enzymes server at URL: <http://afmb.cnrs-mrs.fr/CAZY/index.html>).
In order to be efficient, the digestion of cellulose requires several types of enzymes acting cooperatively. At least three categories of enzymes are necessary to convert cellulose into glucose: endo (1,4)-beta-D-glucanases (EC 3.2.1.4) that cut the cellulose chains at random; cellobiohydrolases (EC 3.2.1.91) which cleave cellobiosyl units from the cellulose chain ends and beta-glucosidases (EC 3.2.1.21) that convert cellobiose and soluble cellodextrins into glucose.
It is an object of the present invention to provide improved proteins having cellulose- or hemicellulose-degrading activity and polynucleotides encoding the proteins. It is an object of the present invention to provide improved proteins having cellulose- or hemicellulose-binding activity and polynucleotides encoding the proteins. The improved proteins may improve the degradation of cell wall material, e.g., cellulose and/or hemicellulose. The proteins may also improve the stability or activity of other enzymes involved in the degradation of plant cell wall material, e.g., biomass.